CMSampleAttachmentKey should be public

[lobrien]

Apple samples use kCMSampleBufferAttachmentKey_CameraIntrinsicMatrix, which AFAICT is not publicly available in Xamarin.iOS

Steps to Reproduce

1. Download Apple sample Tracking the User’s Face in Real Time
2. Unzip and open ViewController.swift
3. Navigate to captureOutput and line 462:

 let cameraIntrinsicData = CMGetAttachment(sampleBuffer, key: kCMSampleBufferAttachmentKey_CameraIntrinsicMatrix, attachmentModeOut: nil)

4. Create a Xamarin.iOS workbook and enter what I believe to be the equivalent code in C#:

		var cameraIntrinsicData = CMAttachmentBearer.GetAttachment(CMSampleAttachmentKey.CameraIntrinsicMatrixKey, sampleBuffer ); // typeof(sampleBuffer) is `CVSampleBuffer`
		

Expected Behavior

The code compiles.

Actual Behavior

The compiler says "CMSampleAttachmentKey is inaccessible due to its protection level."

Workaround

Use the hardcoded value "CameraIntrinsicMatrix"

Environment

=== Visual Studio Community 2017 for Mac (Preview) ===

Version 7.6 Preview (7.6 build 1773)
Installation UUID: 6b94f136-026d-4a5a-bf6d-af2c0d8dc019
Runtime:
Mono 5.12.0.273 (2018-02/f59eac4c0f1) (64-bit)
GTK+ 2.24.23 (Raleigh theme)
Xamarin.Mac 4.4.1.178 (master / eeaeb7e)

Package version: 512000273

=== NuGet ===

Version: 4.3.1.4445

=== .NET Core ===

Runtime: /usr/local/share/dotnet/dotnet
Runtime Versions:
2.1.1
2.0.5
2.0.3
2.0.0
SDK: /usr/local/share/dotnet/sdk/2.1.301/Sdks
SDK Versions:
2.1.301
2.1.4
2.0.3
2.0.0
MSBuild SDKs: /Library/Frameworks/Mono.framework/Versions/5.12.0/lib/mono/msbuild/15.0/bin/Sdks

=== Xamarin.Profiler ===

Version: 1.6.3
Location: /Applications/Xamarin Profiler.app/Contents/MacOS/Xamarin Profiler

=== Xamarin.Android ===

Version: 8.4.0.1 (Visual Studio Community)
Android SDK: /Users/larryobrien/Library/Android/sdk
Supported Android versions:
4.1 (API level 16)
5.0 (API level 21)
6.0 (API level 23)
7.0 (API level 24)
7.1 (API level 25)
8.0 (API level 26)
8.1 (API level 27)

SDK Tools Version: 26.1.1
SDK Platform Tools Version: 25.0.3
SDK Build Tools Version: 27.0.3

Java SDK: /usr
java version "1.8.0_162"
Java(TM) SE Runtime Environment (build 1.8.0_162-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.162-b12, mixed mode)

Android Designer EPL code available here:
https://github.com/xamarin/AndroidDesigner.EPL

=== Xamarin Inspector ===

Version: 1.4.0
Hash: b3f92f9
Branch: master
Build date: Fri, 19 Jan 2018 22:00:34 GMT
Client compatibility: 1

=== Apple Developer Tools ===

Xcode 10.0 (14313.2)
Build 10L213o

=== Xamarin.Mac ===

Version: 4.99.3.344 (Visual Studio Community)
Hash: 0b72651
Branch:
Build date: 2018-08-09 10:37:58-0400

=== Xamarin.iOS ===

Version: 11.99.3.224 (Visual Studio Community)
Hash: 0b72651
Branch: HEAD
Build date: 2018-08-09 10:37:57-0400

=== Build Information ===

Release ID: 706001773
Git revision: e5958ff9015312c81fc3b1b6b6413585575ca869
Build date: 2018-06-19 12:19:27+00
Build branch: release-7.6
Xamarin extensions: 6910f36b43e7d559ff40ae268e73ae26c30a2a6b

=== Operating System ===

Mac OS X 10.13.6
Darwin 17.7.0 Darwin Kernel Version 17.7.0
Thu Jun 21 22:53:14 PDT 2018
root:xnu-4570.71.2~1/RELEASE_X86_64 x86_64

=== Enabled user installed extensions ===

AddinMaker 1.4.2
Larry's F# Templates 2.0.8
MicroFramework 1.0.3
Internet of Things (IoT) development (Preview) 7.5.3

[mandel-macaque]

@lobrien what about the following:

var att = new CMSampleBufferAttachmentSettings (sampleBuffer.GetAttachments (CMAttachmentMode.ShouldNotPropagate));
var matrix = att.CameraIntrinsicMatrix;

I'm not sure about the mode, but you can create the CMSampleBufferAttachmentSettings from the NSDictionary and then used them. (could we have a performance hit?)

[VincentDondain]

I think what @mandel-macaque should work but that's a convoluted way to achieve what you want here. Also there's kinda no point having CMAttachmentBearer's GetAttachment public if it takes a string key but we don't make the keys you can give it public.

It doesn't cost us to open up (remove [Internal]) CMSampleAttachmentKey. It's internal because there's the strong dictionary but it's used in different way for different APIs. We probably did not know at the time that CMAttachmentBearer's GetAttachment wanted CMSampleAttachmentKey. The documentation isn't very precise on that matter: https://developer.apple.com/documentation/coremedia/1470707-cmgetattachment?language=objc

Note: we could make a smart enum out of CMSampleAttachmentKey and a new GetAttachment overload to make it easier to use. The problem is that it might take keys other than CMSampleAttachmentKey. Don't know about that.